Luke Remage-Healey still recalls the day he first suspected that gulf toadfish can listen in on hungry dolphins' calls. Floating above toadfish nests in a research boat, he was recording the mating calls of male toadfish when, `suddenly, they all went quiet.' Puzzled, he realised that he was now recording dolphin calls instead; peering over the side of the boat, his field assistant spotted dolphins hovering over the toadfish nests. Wondering if these two events were linked, Remage-Healey and Andrew Bass set out to see if toadfish can eavesdrop on dolphins(p. 4444).
They called in the assistance of Douglas Nowacek, a marine mammal expert at Florida State University. From previous work, Nowacek knew that dolphins pay attention to the sounds of their prey. But is the reverse also true: are toadfish aware of their predators? To find out, Remage-Healey decided to play dolphin sounds to toadfish and see if the fish became stressed and stopped calling. Nowacek had recordings of the high frequency `whistles' that dolphins use to communicate with each other, as well as the low frequency `pops' that dolphins probably use to locate their lunch, because these sounds can penetrate sandbeds and sea grass, where toadfish like to breed. `Toadfish are low frequency specialists,' Remage-Healey says. `They hear best below 1 kHz,so we suspected that they would be able to hear the dolphins' low frequency pops but not the high frequency whistles.'
Conveniently, toadfish breed in the bay just outside Florida State University's marine lab. Coaxing toadfish out of their nests proved difficult,but Remage-Healey soon discovered that he could catch fish by collecting the shells they nest under. `They defend their nest aggressively, so they hang onto the shell even when it's moving,' he says. Taking his research boat to the toadfish breeding site, Remage-Healey placed each captured toadfish in its own cage and nestled the cages on the seabed in the breeding patch. He then lowered a speaker into the water and played one of four recordings: dolphin pops, dolphin whistles, dolphin pops and whistles or snapping shrimp pops, a common background noise in the bay that shouldn't alarm the toadfish. He recorded each toadfish's calls before, during and after the playback. To see how stressed the fish were after listening to each recording, he took blood samples to measure levels of the stress hormone cortisol.
Remage-Healey was delighted to have his suspicions confirmed. He found that toadfish listening to snapping shrimp pops or dolphin whistles happily kept on calling, but fish that heard dolphin pops or a combination of pops and whistles drastically reduced their calling rates. `This suggests that toadfish can perceive dolphin foraging sounds and respond behaviourally to reduce the chance of being overheard,' he says. But he was really convinced when he saw that the cortisol levels of toadfish that had listened to dolphin pops had shot up compared with those of fish that had listened to shrimp pops; toadfish clearly find dolphin pops more stressful to listen to. `This really cemented the idea that something was happening,' he says.
Remage-Healey is intrigued by this evidence for a co-evolutionary game between a fish and a mammal. For toadfish, there is a clear trade-off between calling to attract a mate and keeping quiet to avoid being eaten, while dolphins face a trade-off between the need to use low frequency pops to locate their dinner and the risk of being overheard by their prey.
